has any one ever tried and impeller as a pump?

As the title says has any one ever tried and impeller as a pump

Cause couldn’t you put the impeller in the reservoir have the motor out side the reservoir

would'nt an impeller kick out more water and save room cause it has to be in the reservoir an u wouldnt get heat from the motor cause its outside of the reservoir?

The impeller needs a housing to work properly, it wouldn't work if it was just spinning freely. Also, a good deal of that heat comes from the kinetic engery of the impeller moving the water.